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Libraries
Python
Libraries
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.
A function is a way to turn a bunch of related
statements into a single "chunk"
Python Libraries
A function is a way to turn a bunch of related
statements into a single "chunk"
– Avoid duplication– Avoid duplication
Python Libraries
A function is a way to turn a bunch of related
statements into a single "chunk"
– Avoid duplication– Avoid duplication
– Make code easier to read
Python Libraries
A function is a way to turn a bunch of related
statements into a single "chunk"
– Avoid duplication– Avoid duplication
– Make code easier to read
A library does the same thing for related functions
Python Libraries
A function is a way to turn a bunch of related
statements into a single "chunk"
– Avoid duplication– Avoid duplication
– Make code easier to read
A library does the same thing for related functions
Hierarchical organization
Python Libraries
A function is a way to turn a bunch of related
statements into a single "chunk"
– Avoid duplication– Avoid duplication
– Make code easier to read
A library does the same thing for related functions
Hierarchical organization
Python Libraries
family library
genus function
species statement
Every Python file can be used as a library
Python Libraries
Every Python file can be used as a library
Use import to load it
Python Libraries
Every Python file can be used as a library
Use import to load it
# halman.py# halman.py
defdefdefdef threshold(signal):
returnreturnreturnreturn 1.0 / sum(signal)
Python Libraries
Every Python file can be used as a library
Use import to load it
# halman.py# halman.py
defdefdefdef threshold(signal):
returnreturnreturnreturn 1.0 / sum(signal)
# program.py
importimportimportimport halman
readings = [0.1, 0.4, 0.2]
printprintprintprint 'signal threshold is', halman.threshold(readings)
Python Libraries
printprintprintprint 'signal threshold is', halman.threshold(readings)
Every Python file can be used as a library
Use import to load it
# halman.py# halman.py
defdefdefdef threshold(signal):
returnreturnreturnreturn 1.0 / sum(signal)
# program.py
importimportimportimport halman
readings = [0.1, 0.4, 0.2]
printprintprintprint 'signal threshold is', halman.threshold(readings)
Python Libraries
printprintprintprint 'signal threshold is', halman.threshold(readings)
$$$$ python program.py
signal threshold is 1.42857
When a module is imported, Python:
Python Libraries
When a module is imported, Python:
1. Executes the statements it contains
Python Libraries
When a module is imported, Python:
1. Executes the statements it contains
2. Creates an object that stores references to2. Creates an object that stores references to
the top-level items in that module
Python Libraries
When a module is imported, Python:
1. Executes the statements it contains
2. Creates an object that stores references to2. Creates an object that stores references to
the top-level items in that module
# noisy.py
printprintprintprint 'is this module being loaded?'
NOISE_LEVEL = 1./3.
Python Libraries
When a module is imported, Python:
1. Executes the statements it contains
2. Creates an object that stores references to2. Creates an object that stores references to
the top-level items in that module
# noisy.py
printprintprintprint 'is this module being loaded?'
NOISE_LEVEL = 1./3.
Python Libraries
>>>>>>>>>>>> import noisy
is this module being loaded?
When a module is imported, Python:
1. Executes the statements it contains
2. Creates an object that stores references to2. Creates an object that stores references to
the top-level items in that module
# noisy.py
printprintprintprint 'is this module being loaded?'
NOISE_LEVEL = 1./3.
Python Libraries
>>>>>>>>>>>> import noisy
is this module being loaded?
>>>>>>>>>>>> print noisy.NOISE_LEVEL
0.33333333
Each module is a namespace
Python Libraries
Each module is a namespace
function
Python Libraries
Each module is a namespace
modulemodule
function
Python Libraries
Each module is a namespace
global
modulemodule
function
Python Libraries
Each module is a namespace
global
module
# module.py
NAME = 'Transylvania'
defdefdefdef func(arg):
module
function
Python Libraries
defdefdefdef func(arg):
returnreturnreturnreturn NAME + ' ' + arg
Each module is a namespace
global
module
# module.py
NAME = 'Transylvania'
defdefdefdef func(arg):
module
function
>>>>>>>>>>>> NAME = 'Hamunaptra'
Python Libraries
defdefdefdef func(arg):
returnreturnreturnreturn NAME + ' ' + arg
Each module is a namespace
global
module
# module.py
NAME = 'Transylvania'
defdefdefdef func(arg):
module
function
>>>>>>>>>>>> NAME = 'Hamunaptra'
>>>>>>>>>>>> import module
Python Libraries
defdefdefdef func(arg):
returnreturnreturnreturn NAME + ' ' + arg
Each module is a namespace
global
module
# module.py
NAME = 'Transylvania'
defdefdefdef func(arg):
module
function
>>>>>>>>>>>> NAME = 'Hamunaptra'
>>>>>>>>>>>> import module
>>>>>>>>>>>> print module.func('!!!')
Transylvania !!!
Python Libraries
defdefdefdef func(arg):
returnreturnreturnreturn NAME + ' ' + arg
Transylvania !!!
Python comes with many standard libraries
Python Libraries
Python comes with many standard libraries
>>> import math
Python Libraries
Python comes with many standard libraries
>>> import math
>>> >>> >>> >>> printprintprintprint math.sqrt(2)
1.41421356237309511.4142135623730951
Python Libraries
Python comes with many standard libraries
>>> import math
>>> >>> >>> >>> printprintprintprint math.sqrt(2)
1.41421356237309511.4142135623730951
>>> >>> >>> >>> print print print print math.hypot(2, 3) # sqrt(x**2 + y**2)
3.6055512754639891
Python Libraries
Python comes with many standard libraries
>>> import math
>>> >>> >>> >>> printprintprintprint math.sqrt(2)
1.41421356237309511.4142135623730951
>>> >>> >>> >>> print print print print math.hypot(2, 3) # sqrt(x**2 + y**2)
3.6055512754639891
>>> >>> >>> >>> print print print print math.e, math.pi # as accurate as possible
2.7182818284590451 3.1415926535897931
Python Libraries
Python also provides a help function
Python Libraries
Python also provides a help function
>>> >>> >>> >>> import import import import math
>>> >>> >>> >>> help(math)Help on module math:Help on module math:NAME
mathFILE
/usr/lib/python2.5/lib-dynload/math.soMODULE DOCS
http://www.python.org/doc/current/lib/module-math.htmlDESCRIPTION
This module is always available. It provides access to themathematical functions defined by the C standard.
Python Libraries
mathematical functions defined by the C standard.FUNCTIONS
acos(...)acos(x)Return the arc cosine (measured in radians) of x.
⋮
And some nicer ways to do imports
Python Libraries
And some nicer ways to do imports
>>> >>> >>> >>> fromfromfromfrom math importimportimportimport sqrt
>>> >>> >>> >>> sqrt(3)
1.73205080756887721.7320508075688772
Python Libraries
And some nicer ways to do imports
>>> >>> >>> >>> fromfromfromfrom math importimportimportimport sqrt
>>> >>> >>> >>> sqrt(3)
1.73205080756887721.7320508075688772
>>>>>>>>>>>> fromfromfromfrom math importimportimportimport hypot asasasas euclid
>>>>>>>>>>>> euclid(3, 4)
5.0
Python Libraries
And some nicer ways to do imports
>>> >>> >>> >>> fromfromfromfrom math importimportimportimport sqrt
>>> >>> >>> >>> sqrt(3)
1.73205080756887721.7320508075688772
>>>>>>>>>>>> fromfromfromfrom math importimportimportimport hypot asasasas euclid
>>>>>>>>>>>> euclid(3, 4)
5.0>>> >>> >>> >>> fromfromfromfrom math importimportimportimport *
>>> >>> >>> >>> sin(pi)
1.2246063538223773e-16>>>>>>>>>>>>
Python Libraries
>>>>>>>>>>>>
And some nicer ways to do imports
>>> >>> >>> >>> fromfromfromfrom math importimportimportimport sqrt
>>> >>> >>> >>> sqrt(3)
1.73205080756887721.7320508075688772
>>>>>>>>>>>> fromfromfromfrom math importimportimportimport hypot asasasas euclid
>>>>>>>>>>>> euclid(3, 4)
5.0>>> >>> >>> >>> fromfromfromfrom math importimportimportimport *
>>> >>> >>> >>> sin(pi)
1.2246063538223773e-16>>>>>>>>>>>>
Generally a bad idea
Python Libraries
>>>>>>>>>>>>
And some nicer ways to do imports
>>> >>> >>> >>> fromfromfromfrom math importimportimportimport sqrt
>>> >>> >>> >>> sqrt(3)
1.73205080756887721.7320508075688772
>>>>>>>>>>>> fromfromfromfrom math importimportimportimport hypot asasasas euclid
>>>>>>>>>>>> euclid(3, 4)
5.0>>> >>> >>> >>> fromfromfromfrom math importimportimportimport *
>>> >>> >>> >>> sin(pi)
1.2246063538223773e-16>>>>>>>>>>>>
Generally a bad idea
Someone could add to
the library after you
Python Libraries
>>>>>>>>>>>>the library after you
start using it
Almost every program uses the sys library
Python Libraries
Almost every program uses the sys library
>>> >>> >>> >>> importimportimportimport sys
Python Libraries
Almost every program uses the sys library
>>> >>> >>> >>> importimportimportimport sys
>>> >>> >>> >>> printprintprintprint sys.version
2.7 (r27:82525, Jul 4 2010, 09:01:59)2.7 (r27:82525, Jul 4 2010, 09:01:59)
[MSC v.1500 32 bit (Intel)]
Python Libraries
Almost every program uses the sys library
>>> >>> >>> >>> importimportimportimport sys
>>> >>> >>> >>> printprintprintprint sys.version
2.7 (r27:82525, Jul 4 2010, 09:01:59)2.7 (r27:82525, Jul 4 2010, 09:01:59)
[MSC v.1500 32 bit (Intel)]>>>>>>>>>>>> printprintprintprint sys.platform
win32
Python Libraries
Almost every program uses the sys library
>>> >>> >>> >>> importimportimportimport sys
>>> >>> >>> >>> printprintprintprint sys.version
2.7 (r27:82525, Jul 4 2010, 09:01:59)2.7 (r27:82525, Jul 4 2010, 09:01:59)
[MSC v.1500 32 bit (Intel)]>>>>>>>>>>>> printprintprintprint sys.platform
win32>>>>>>>>>>>> printprintprintprint sys.maxint
2147483647
Python Libraries
Almost every program uses the sys library
>>> >>> >>> >>> importimportimportimport sys
>>> >>> >>> >>> printprintprintprint sys.version
2.7 (r27:82525, Jul 4 2010, 09:01:59)2.7 (r27:82525, Jul 4 2010, 09:01:59)
[MSC v.1500 32 bit (Intel)]>>>>>>>>>>>> printprintprintprint sys.platform
win32>>>>>>>>>>>> printprintprintprint sys.maxint
2147483647
>>> >>> >>> >>> printprintprintprint sys.path
['',
Python Libraries
['','C:\\WINDOWS\\system32\\python27.zip',
'C:\\Python27\\DLLs', 'C:\\Python27\\lib','C:\\Python27\\lib\\plat-win',
'C:\\Python27', 'C:\\Python27\\lib\\site-packages']
sys.argv holds command-line arguments
Python Libraries
sys.argv holds command-line arguments
Script name is sys.argv[0]
Python Libraries
sys.argv holds command-line arguments
Script name is sys.argv[0]
# echo.py# echo.py
importimportimportimport sys
forforforfor i inininin range(len(sys.argv)):
printprintprintprint i, '"' + sys.argv[i] + '"'
Python Libraries
sys.argv holds command-line arguments
Script name is sys.argv[0]
# echo.py# echo.py
importimportimportimport sys
forforforfor i inininin range(len(sys.argv)):
printprintprintprint i, '"' + sys.argv[i] + '"'
$ $ $ $ python echo.py
0 echo.py
$$$$
Python Libraries
$$$$
sys.argv holds command-line arguments
Script name is sys.argv[0]
# echo.py# echo.py
importimportimportimport sys
forforforfor i inininin range(len(sys.argv)):
printprintprintprint i, '"' + sys.argv[i] + '"'
$ $ $ $ python echo.py
0 echo.py
$$$$ python echo.py first second
Python Libraries
$$$$ python echo.py first second
0 echo.py
1 first
2 second
$$$$
sys.stdin is standard input (e.g., the keyboard)
Python Libraries
sys.stdin is standard input (e.g., the keyboard)
sys.stdout is standard output (e.g., the screen)
Python Libraries
sys.stdin is standard input (e.g., the keyboard)
sys.stdout is standard output (e.g., the screen)
sys.stderr is standard error (usually also the screen)sys.stderr is standard error (usually also the screen)
Python Libraries
sys.stdin is standard input (e.g., the keyboard)
sys.stdout is standard output (e.g., the screen)
sys.stderr is standard error (usually also the screen)sys.stderr is standard error (usually also the screen)
See the Unix shell lecture for more information
Python Libraries
# count.py
importimportimportimport sys
ifififif len(sys.argv) == 1:
count_lines(sys.stdin)
else:else:else:else:else:else:else:else:
rd = open(sys.argv[1], 'r')
count_lines(rd)
rd.close()
Python Libraries
# count.py
importimportimportimport sys
ifififif len(sys.argv) == 1:
count_lines(sys.stdin)
else:else:else:else:else:else:else:else:
rd = open(sys.argv[1], 'r')
count_lines(rd)
rd.close()
Python Libraries
# count.py
importimportimportimport sys
ifififif len(sys.argv) == 1:
count_lines(sys.stdin)
else:else:else:else:else:else:else:else:
rd = open(sys.argv[1], 'r')
count_lines(rd)
rd.close()
Python Libraries
# count.py
importimportimportimport sys
ifififif len(sys.argv) == 1:
count_lines(sys.stdin)
else:else:else:else:else:else:else:else:
rd = open(sys.argv[1], 'r')
count_lines(rd)
rd.close()
$$$$ python count.py < a.txt
48
$$$$
Python Libraries
$$$$
# count.py
importimportimportimport sys
ifififif len(sys.argv) == 1:
count_lines(sys.stdin)
else:else:else:else:else:else:else:else:
rd = open(sys.argv[1], 'r')
count_lines(rd)
rd.close()
$$$$ python count.py < a.txt
48
$$$$ python count.py b.txt
Python Libraries
$$$$ python count.py b.txt
227
$$$$
The more polite way
'''Count lines in files. If no filename arguments given,
read from standard input.'''
importimportimportimport sys
defdefdefdef count_lines(reader):
'''Return number of lines in text read from reader.'''
returnreturnreturnreturn len(reader.readlines())
ifififif __name__ == '__main__':
Python Libraries
ifififif __name__ == '__main__':
...as before...
The more polite way
'''Count lines in files. If no filename arguments given,
read from standard input.'''
importimportimportimport sys
defdefdefdef count_lines(reader):
'''Return number of lines in text read from reader.'''
returnreturnreturnreturn len(reader.readlines())
ifififif __name__ == '__main__':
Python Libraries
ifififif __name__ == '__main__':
...as before...
The more polite way
'''Count lines in files. If no filename arguments given,
read from standard input.'''
importimportimportimport sys
defdefdefdef count_lines(reader):
'''Return number of lines in text read from reader.'''
returnreturnreturnreturn len(reader.readlines())
ifififif __name__ == '__main__':
Python Libraries
ifififif __name__ == '__main__':
...as before...
If the first statement in a module or function is
a string, it is saved as a docstring
Python Libraries
If the first statement in a module or function is
a string, it is saved as a docstring
Used for online (and offline) helpUsed for online (and offline) help
Python Libraries
If the first statement in a module or function is
a string, it is saved as a docstring
Used for online (and offline) helpUsed for online (and offline) help
# adder.py
'''Addition utilities.'''
defdefdefdef add(a, b):
'''Add arguments.'''
returnreturnreturnreturn a+b
Python Libraries
returnreturnreturnreturn a+b
If the first statement in a module or function is
a string, it is saved as a docstring
Used for online (and offline) helpUsed for online (and offline) help
# adder.py
'''Addition utilities.'''
defdefdefdef add(a, b):
'''Add arguments.'''
returnreturnreturnreturn a+b
>>> >>> >>> >>> import adder
>>>>>>>>>>>> help(adder)
NAME
adder - Addition utilities.
FUNCTIONSadd(a, b)
Python Libraries
returnreturnreturnreturn a+b add(a, b)Add arguments.
>>>>>>>>>>>>
If the first statement in a module or function is
a string, it is saved as a docstring
Used for online (and offline) helpUsed for online (and offline) help
# adder.py
'''Addition utilities.'''
defdefdefdef add(a, b):
'''Add arguments.'''
returnreturnreturnreturn a+b
>>> >>> >>> >>> import adder
>>>>>>>>>>>> help(adder)
NAME
adder - Addition utilities.
FUNCTIONSadd(a, b)
Python Libraries
returnreturnreturnreturn a+b add(a, b)Add arguments.
>>>>>>>>>>>> help(adder.add)
add(a, b)
Add arguments.>>>>>>>>>>>>
When Python loads a module, it assigns a value
to the module-level variable __name__
Python Libraries
When Python loads a module, it assigns a value
to the module-level variable __name__
main program
'__main__'
Python Libraries
When Python loads a module, it assigns a value
to the module-level variable __name__
main program loaded as library
'__main__' module name
Python Libraries
When Python loads a module, it assigns a value
to the module-level variable __name__
main program loaded as library
'__main__' module name
...module definitions...
Python Libraries
ifififif __name__ == '__main__':
...run as main program...
When Python loads a module, it assigns a value
to the module-level variable __name__
main program loaded as library
'__main__' module name
...module definitions... Always executed
Python Libraries
ifififif __name__ == '__main__':
...run as main program...
When Python loads a module, it assigns a value
to the module-level variable __name__
main program loaded as library
'__main__' module name
...module definitions... Always executed
Python Libraries
ifififif __name__ == '__main__':
...run as main program... Only executed when
file run directly
# stats.py
'''Useful statistical tools.'''
defdefdefdef average(values):
'''Return average of values or None if no data.''''''Return average of values or None if no data.'''
ifififif values:
returnreturnreturnreturn sum(values) / len(values)
else:else:else:else:
return Nonereturn Nonereturn Nonereturn None
if if if if __name__ == '__main__':
printprintprintprint 'test 1 should be None:', average([])
Python Libraries
printprintprintprint 'test 1 should be None:', average([])
printprintprintprint 'test 2 should be 1:', average([1])
printprintprintprint 'test 3 should be 2:', average([1, 2, 3])
# test-stats.py
from from from from stats importimportimportimport average
printprintprintprint 'test 4 should be None:', average(set())
printprintprintprint 'test 5 should be -1:', average({0, -1, -2})
Python Libraries
# test-stats.py
from from from from stats importimportimportimport average
printprintprintprint 'test 4 should be None:', average(set())
printprintprintprint 'test 5 should be -1:', average({0, -1, -2})
$$$$ python stats.py
test 1 should be None: None
test 2 should be 1: 1
test 3 should be 2: 2
$$$$
Python Libraries
# test-stats.py
from from from from stats importimportimportimport average
printprintprintprint 'test 4 should be None:', average(set())
printprintprintprint 'test 5 should be -1:', average({0, -1, -2})
$$$$ python stats.py
test 1 should be None: None
test 2 should be 1: 1
test 3 should be 2: 2
$$$$ python test-stats.py
test 4 should be None: None
test 5 should be -1: -1
Python Libraries
test 5 should be -1: -1
$$$$
October 2010
created by
Greg Wilson
October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.
